Abstract:

A corrected channel estimating apparatus is disclosed, which is capable of
removing side-lobe components mutually leaking into transmission paths
from channel estimates to increase the accuracy. A channel estimation
unit calculates channel estimates for a plurality of transmission paths
due to a signal that arrives through multipath. A channel estimate
correction unit removes side-lobe components mutually leaking into the
transmission paths from the channel estimates calculated by the channel
estimation unit to correct the channel estimates. In this way, the
corrected channel estimating apparatus estimates transmission paths of a
multipath.

Claims:

1-9. (canceled)

10. A corrected channel estimating apparatus for estimating transmission
paths of a multipath, comprising:a channel estimation unit for
calculating channel estimates for a plurality of transmission paths from
a signal arriving through a multipath; anda channel estimate correction
unit for removing side-lobe components mutually leaking into the
transmission paths, from the channel estimates calculated by said channel
estimation unit, to correct the channel estimates.

11. The corrected channel estimating apparatus according to claim 10,
wherein said channel estimate correction unit calculates values, which
exclude the side-lobe components mutually leaking into the transmission
paths from the channel estimates, by using the channel estimates for a
plurality of the transmission paths calculated by said channel estimation
unit and by using timing differences between the plurality of respective
transmission paths.

12. The corrected channel estimating apparatus according to claim 10,
wherein said channel estimate correction unit further comprising a path
selection unit for selecting transmission paths which present a timing
difference less than a predetermined value from any of the other
transmission paths, as eligible for correction.

13. The corrected channel estimating apparatus according to claim 11 or
12, wherein said channel estimate correction unit generates coefficients
by using timing differences between the plurality of respective
transmission paths, and removes the leaking side-lobe components from the
channel estimates by calculating the sums of products of the coefficients
and the channel estimates calculated by said channel estimation unit.

14. The corrected channel estimating apparatus according to claim 13,
wherein said channel estimate correction unit further comprises a path
change determination unit for detecting a change in timing of the
transmission path, wherein said channel estimate correction unit
regenerates said coefficients when the path change determination unit
detects the change in path timing.

15. The corrected channel estimating apparatus according to claim 13,
wherein said channel estimate correction unit comprises:side-lobe
correlation matrix generation unit for generating a correlation matrix
indicative of the amounts of leaking side-lobe components due to the
timing differences between the plurality of respective transmission
paths, an inverse matrix processing unit for calculating an inverse
matrix of the correlation matrix generated by said side-lobe correlation
matrix generation unit, and a decorrelate processing unit for removing
the leaking side-lobe components from the channel estimates by
calculating the sums of products of the inverse matrix calculated by said
inverse matrix processing unit and the channel estimates calculated by
said channel estimation unit.

16. The corrected channel estimating apparatus according to claim 15,
wherein said channel estimate correction unit further comprises a path
change determination unit for detecting a change in timing of the
transmission path, wherein said side-lobe correlation matrix generation
unit generates the correlation matrix when said path change determination
unit detects the change in path timing.

17. A CDMA receiving apparatus for reception of a CDMA received signal,
said CDMA receiving apparatus comprising:a channel estimation unit for
calculating channel estimates for a plurality of transmission paths from
a signal arriving through a multipath;a channel estimate correction unit
for removing side-lobe components mutually leaking into the transmission
paths, from the channel estimates calculated by said channel estimation
unit, to correct the channel estimates; anda data despreading unit for
despreading the signal.

18. The CDMA receiving apparatus according to claim 17,further comprising
an equalizer unit using equalization weight that calculated from channel
estimates corrected by said channel estimate correction unit,herein said
data despreading unit despreads the signal generated by said equalizer
unit.

19. The CDMA receiving apparatus according to claim 18, wherein said
channel estimate correction unit calculates values which exclude the
side-lobe components mutually leaking into the transmission paths from
the channel estimates, by using the channel estimates for a plurality of
the transmission paths calculated by said channel estimation unit and by
using timing differences between the plurality of respective transmission
paths.

20. A corrected channel estimating method for estimating transmission
paths of a multipath, said method comprising:a first step of calculating
channel estimates for a plurality of transmission paths from a signal
arriving through a multipath;a second step of removing side-lobe
components mutually leaking into the transmission paths, from the
calculated channel estimates, to correct the channel estimates.

21. The corrected channel estimating method according to claim 20, wherein
said second step includes calculating values which exclude the side-lobe
components mutually leaking into the transmission paths from the channel
estimates, by using the channel estimates for a plurality of the
transmission paths calculated at said first step and by using timing
differences between the plurality of respective transmission paths.

22. A corrected channel estimating apparatus for estimating transmission
paths of a multipath, comprising:a channel estimation means for
calculating channel estimates for a plurality of transmission paths from
a signal arriving through a multipath; anda channel estimate correction
means for removing side-lobe components mutually leaking into the
transmission paths, from the channel estimates calculated by said channel
estimation means, to correct the channel estimates.

23. A CDMA receiving apparatus for reception of a CDMA received signal,
said CDMA receiving apparatus comprising:a channel estimation means for
calculating channel estimates for a plurality of transmission paths from
a signal arriving through a multipath;a channel estimate correction means
for removing side-lobe components mutually leaking into the transmission
paths, from the channel estimates calculated by said channel estimation
means, to correct the channel estimates;an equalizer means using
equalization weight that calculated from channel estimates corrected by
said channel estimate correction means; anda data despreading means for
despreading the signal generated by said equalizer means.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a channel estimating apparatus and
a CDMA receiving apparatus using the same, and more particularly, to a
corrected channel estimating apparatus which improves the accuracy of
channel estimation, and a CDMA receiving apparatus using the same.

BACKGROUND ART

[0002]A DS-CDMA (Direct Sequence--Code Division Multiple Access) scheme
can effectively suppress interference from other cells in a multi-cell
environment. Thus, the DS-CDMA scheme can accomplish so-called one-cell
repetition frequency assignment which uses the same frequency in adjacent
cells, so that it is suitably used as a wireless access scheme for mobile
communications.

[0003]The CDMA scheme can also achieve path diversity effects by
separating transmission paths of multipath and rake combining them when
de-spreading is performed.

[0004]In recent years, however, data communication traffic has been
increased in addition to voice communications in mobile communications,
so that larger capacities and higher speeds are desired. Then, multi-code
based transmissions are required for enabling larger capacities and
higher speeds. However, when the multi-code transmission is performed in
a CDMA scheme, a problem arises in that the effective spreading gain is
reduced which compromises the effect of suppressing multipath
interference.

[0005]Accordingly, investigations have been made on equalization performed
on a transmission path to recover orthogonality of multi-codes before
despreading. Also, a variety of methods have been proposed as equalizing
methods therefor. For example, there is a simple method which employs a
linear filter (see, for example, Kawamura, Kishiyama, Higuchi, and
Sawahashi, "Comparison in Characteristics between Multi-Path Interference
Canceler and Chip Equalizer in Consideration of Removal of Other Cell
Interference in HSDPA," Technical Report RCS2002-38, April 2002). There
is also an equalizing method which employs frequency conversion (see, for
example, D. Falconer et al, "Frequency Domain Equalization for
Single-Carrier Broadband Wireless System," IEEE Commun. Mag., vol. 40,
no. 4, pp. 58-66, April 2002).

[0009]Pilot despreading unit 1031-103L despreads a pilot signal
within a CDMA signal based on the timing of a path assigned thereto by
path search unit 101.

[0010]Symbol averaging unit 1041-104L receives a despread pilot
signal from pilot despreading unit 1031-103L on a path-by-path
basis, and averages the despread pilot signal over a plurality of symbols
to calculate channel estimates on a path-by-path basis.

[0015]FIG. 2 is a diagram illustrating an exemplary configuration of an
equalization filter. Referring to FIG. 2, equalization filter 107, which
is an FIR (Finite Impulse Response) filter, is composed of delays
211-21N-1, multipliers 221-22N, and adder 23.

[0016]Equalization filter 107 delays a received signal by delays
211-21N-1 in units of samples to generate outputs of respective
taps, and multiplies the outputs of the respective taps by weights
w0-wN-1 by multipliers 221-22N, and adds outputs of
respective multipliers 221-22N by adder 23.

[0018]A corrected channel estimating apparatus separates transmission
paths of multipath by multiplying a received signal by a complex
conjugate of a known pilot signal to find channel estimates on a
path-by-path basis.

[0019]Generally, since a signal of a communication scheme employing band
limitation, such as CDMA, is limited in band by a roll-off filter, its
impulse response has an extended side-lobe component. When multipath
signal is separated by despreading, components at different timings are
suppressed by a spreading gain if the spreading gain is sufficient.
However, side-lobe components at the same timing are not suppressed by
the despreading but leak into channel estimates to compromise the
accuracy of the channel estimates, thus making it difficult to separate
the paths when the paths are in close proximity.

[0020]FIG. 3 is a schematic diagram showing an exemplary relationship
between the impulse response of a filter and multipath. In the example of
FIG. 3, three paths, i.e., path 1 at timing τ0, path 2 at timing
τ1, and path 3 at timing τ2 exist in close proximity.
For this reason, a side-lobe component of path 1 appears at timings
τ1, τ2 of path 2, path 3. This side-lobe component of
path 1 leaks into channel estimates of paths 2, 3 to compromise the
accuracy of the channel estimates of paths 2, 3:

h1, h2 [Equation 5]

[0021]Also, similar to this, side-lobe components of paths 2, 3 leak into
channel estimates of other paths to compromise the accuracy of the
channel estimates.

[0022]Therefore, when timings of respective paths are in close proximity
to one another, the conventional channel estimating apparatus fails to
completely separate the respective paths, even if a sufficient processing
gain can be provided, so that the accuracy of channel estimates is
compromised due to the influence of the leak.

[0024]It is an object of the present invention to provide a channel
estimating apparatus which is capable of calculating a corrected channel
estimate, and a CDMA receiving apparatus using the same. It should be
understood that the channel estimating apparatus provided by the present
invention produces similar effects, as well, when it is employed in the
receiving apparatus of different communication schemes which use band
limitation.

[0025]To achieve the above object, a channel estimating apparatus of the
present invention comprises a channel estimation unit for estimating
transmission paths of multipath, and a channel estimate correction unit.

[0026]The channel estimation unit calculates channel estimates for a
plurality of transmission paths from a signal which arrives through
multipath. The channel estimate correction unit removes side-lobe
components mutually leaking into the transmission paths from the channel
estimates calculated by the channel estimation unit to correct the
channel estimates.

[0027]Thus, according to the present invention, since the channel estimate
correction unit removes the side-lobe components mutually leaking into
the transmission paths from the respective channel estimates calculated
by the channel estimation unit, the transmission paths can be estimated
with high accuracy.

[0035]Path search unit 1 generates the delay profile of a transmission
path using a pilot signal multiplexed on a received signal, and detects a
plurality of paths which present higher1 reception levels from the
delay profile.

[0037]Pilot despreading unit 31-3L multiplies a received signal
by a complex conjugate of a pilot signal based on the timing of a path
assigned thereto by path search unit 1.

[0038]Symbol averaging unit 41-4L receives a despread pilot
signal from pilot despreading unit 31-3L on a path-by-path
basis, and averages the despread pilot signal over a plurality of symbols
to calculate a channel estimate on a path-by-path basis.

[0040]Path selection unit 6 selects, from among a plurality of paths
detected by path search unit 1, paths which have timing differences from
all other paths equal to or larger than a predetermined value as not
eligible for correction, and paths which have timing differences from any
of other paths smaller than the predetermined value as eligible for
correction. This timing difference is the difference between timings at
which signals of respective paths included in a multi-path arrive. When
paths are separated by sufficient intervals, the influence of side-lobe
components can be neglected, so that the amount of processing is reduced
by selecting paths which need no correction, as not eligible.

[0041]Path change determination unit 7 determines whether or not each path
detected by path search unit 1 and selected by path selection unit 6 has
changed in timing from the preceding detection.

[0042]Side-lobe correlation matrix generation unit 8 generates a
correlation matrix, from the result of the determination made by the path
change determination unit 7, that is indicative of amounts of leak
side-lobe components, due to the timing differences between respective
paths, when paths have changed in timing.

[0043]For example, when three paths exist in close proximity as shown in
FIG. 3, side-lobe correlation matrix R is represented by Equation (3):

ττ ττ ττ ττ ττ ττ
##EQU00002##

where hRC(t) is the impulse response of a roll-off filter, which is
represented by Equation (4):

ππ π α α ##EQU00003##

where is a roll-off rate, and Tc is a tip cycle.

[0044]As can be understood from Equation (3) and Equation (4), side-lobe
correlation matrix R depends only on timing differences between the
paths, but does not depend on channel estimates:

hi [Equation 8]

[0045]Thus, side-lobe correlation matrix R is needed to be regenerated
when a change in path timing occurs.

[0046]The channel estimates,

hi [Equation 9]

which are outputs of channel estimation units 21-2L, are sums of
products of elements of side-lobe correlation matrix R and actual
transmission path values hi, and are represented by Equation (5):

##EQU00004##

[0047]Inverse matrix processing unit 9 finds the inverse matrix of
side-lobe correlation matrix R. Inverse matrix processing unit 9 may
employ a method which involves a reduced amount of processing, such as an
LU decomposition method, a Cholesky decomposition method or the like.

[0048]Decorrelate processing unit 10 calculates the sums of products of
elements of inverse matrix R-1 of R and channel estimates:

[0049]While a reduction in power consumption is important in a wireless
mobile device, the processing amount of inverse matrix processing 9 is
not small in this exemplary embodiment. Generally, however, since a path
timing changing speed is lower than a fading varying speed, a path search
cycle can be set longer than a channel estimation cycle. In doing so, it
is possible to reduce the processing amount of inverse matrix processing
9 per channel estimation cycle to an allowable range or less.

[0050]Also, in this exemplary embodiment, path selection unit 6 narrows
down part of the paths for which the side-lobe correlation matrix is
generated, and side-lobe correlation matrix generation unit 8 generates
the side-lobe correlation matrix only when a change in path timing is
detected by path change detection unit 7, thereby allowing for a
reduction in the processing amount. However, path selection unit 6 and
path change determination unit 7 are not essential components, and can be
omitted if the processing amount does not need to be reduced.

[0052]In this event, in channel estimate correction unit 5, side-lobe
correlation matrix generation unit 8 generates the correlation matrix
indicative of side-lobe component leak coefficients due to timing
differences between respective paths which detected by path search unit
1, independently of the channel estimates, inverse matrix calculation
unit 9 calculates the inverse matrix of the correlation matrix, and
decorrelate processing unit 10 removes leaking side-lobe components from
the channel estimates using the inverse matrix. Accordingly, the
correlation matrix for use in removing the leaks need not be processed
each time the channel estimates change.

[0053]Also, in channel estimate correction unit 5, path selection unit 6
excludes those paths that have timing differences equal to or larger than
a predetermined value from other paths, and that are less affected by
leaking side-lobes from paths eligible for correction, so that the
processing amount can be reduced.

[0054]Also, since channel estimate correction unit 5 determines a change
in path timing in path change determination unit 7, and calculates the
correlation matrix only when a change is found in path timing, the
processing amount can be reduced.

[0055]A CDMA receiving apparatus is shown as another exemplary embodiment
of the present invention.

[0061]Equalization weight calculation unit 12 receives a channel estimate
of each path corrected by channel estimate correction unit 5, and
calculates weights used by equalization filter 13. There are a variety of
methods of calculating weights W, including, for example, a method using
MMSE. With the use of MMSE, weights can be calculated by Equation (1).

[0062]Equalization filter 13 equalizes a signal through filtering using a
row vector at the center of weights W calculated by equalization weight
calculation unit 12 as tap weights. An example of equalization filtering
has been shown in FIG. 2. Referring to FIG. 2, equalization filter 107,
which is an FIR filter, is composed of delays 211-21N-1,
multipliers 221-22N, and adder 23.

[0063]Equalization filter 13 delays a received signal by delays
211-21N-1 in units of samples to generate outputs of respective
taps, and multiplies the outputs of the respective taps by weights
w0-wN-1 by multipliers 221-22N, and adds outputs of
respective multipliers 221-22N by adder 23.

[0065]As described above, according to this exemplary embodiment, channel
estimation units 21-2L find respective channel estimates, and
channel estimate correction unit 5 removes side-lobe components mutually
leaking into the respective paths, equalizer unit 11 equalizes signals
with accurate weights calculated using accurate channel estimates from
which the leaks have been removed, and data despreading unit 14 despreads
a signal from equalizer unit 11, so that accurate received data can be
reproduced.

[0066]In this exemplary embodiment, equalizer unit 11 performs
equalization filtering as signal processing in a time domain, but the
present invention is not so limited. Equalizer unit 11 may also perform
the weight calculation and equalization filtering as signal processing in
a frequency domain, in which case similar advantages can be also produced
to those of the exemplary embodiment.

[0067]Also, while the exemplary embodiment has illustrated a CDMA
receiving apparatus which is assumed to comprise one transmission and one
reception antenna, the present invention is not limited to this
configuration. As another example, the present invention can also be
applied to an MIMO (Multiple Input Multiple Output) system which
comprises a plurality of transmission and reception antennas,
respectively.

[0068]Also, the CDMA receiving apparatus of the exemplary embodiment can
be applied to any base station wireless device and a mobile station
wireless device in a mobile communications system.